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What's Whipping Up the Severe Weather?

What's Whipping Up the Severe Weather? A classic Nor’easter plowed up the East Coast of the United States on January 12, 2011, dumping heavy snow on New England states for the third time in three weeks. The Moderate Resolution Imaging Spectroradiometer on NASA’s Terra satellite took this image. Image credit: NASA/GSFC More about this image

January 14, 2011

Devastating floods in Australia. Crippling snow storms along the eastern seaboard of the United States and in Europe.  Torrential rains in South America and heavier than usual rain in California. Do these severe weather conditions have a common origin? The answer is yes and no: Two different conditions are driving these unusually strong weather patterns.

The first condition is called La Niña. It begins in the Pacific Ocean but it can impact weather patterns around the world. La Niña brings unusually cold ocean temperatures in the central Pacific around the equator. Among other things, this cooler ocean water typically results in drier than normal conditions in the U.S. Southwest, Central Plains and Southeast. The Pacific Northwest tends to be wetter than normal. La Niña also usually brings heavier rain in South America, the tropics and parts of Australia.

Deadly Force of La Niña

The current La Niña is particularly strong and fierce. According to Bill Patzert, a climatologist at NASA's Jet Propulsion Laboratory, "This is one of the strongest La Niña events in the past 50 years. Impacts include heavy rains and flooding, which has damaged crops and flooded mines in Australia and Asia. It also has resulted in flooding in northern South America and drought conditions in Argentina." These impacts were anticipated months ago. But not as strong as we are seeing.

The current La Niña’s toll on Australia and South America has been devastating. In the state of Queensland, Australia, the Associated Press reports that relentless flooding has left 26 people dead and more than 30,000 homes and businesses flooded. They also report that in Brazil, authorities say mudslides and flooding have killed more than 500 people in mountain towns north of Rio. At the end of 2010, CNN reported that in Columbia, South America, more than 300 people died due to torrential rains that have engulfed the country for several months.

Arctic Oscillation

A second factor driving the weather is a condition called the Arctic Oscillation. At certain times an Arctic Oscillation causes cold air from near the North Pole to reach far into the lower latitudes. This has been the case since November 2010. This oscillation seems to overpower the effects of La Niña in some regions, particularly the United States. Some climatologists think the Arctic Oscillation has caused the big snow storms and rain in areas  - like California – that are normally dry during La Niña.

"When an Arctic Oscillation (AO) gets this strong," says Patzert, "It definitely overwhelms the effects of La Niña in the mid latitudes of the northern hemisphere." This helps explain the heavy snow in Europe and the U.S. eastern seaboard. But the AO only lasts a few weeks, so La Niña impacts could kick in for most of the winter. Stay tuned! La Niña could set her sights on the United States in the coming weeks.

La Niña tends to last approximately 9 to 12 months, but it can sometimes last more than one year. NASA and other  agencies track La Niña and other long-term weather trends with satellites, buoys and research ships. You can learn more about studying these trends on and .